This invention relates to secondary curing of printing plates fabricated from liquid polymer with the liquid polymer having been first selectively photocured to provide the printing pattern.
It is known by those skilled in the art that certain photopolymeric curing compositions when cured with actinic light under atmospheric conditions, e.g. with air in contact with the surface of the composition, results in a partially cured material having a tacky surface. Several methods have been proposed to overcome this drawback. For example, acrylic and methacrylic compositions can be irradiated in a nitrogen atmosphere which results in a tack-free cured solid product. See U.S. Pat. No. 3,924,023. However, the use of nitrogen per se has the drawback in that, although it excludes oxygen from the surface of the material to be cured, it is necessary to use large amounts of nitrogen. Another method used to overcome the air inhibition during a polymerization of acrylate and methacrylate polymers is the employment of polythiols along with the acrylates and methacrylates in the system to be irradiated. See U.S. Pat. No. 4,008,341. Yet another method of excluding oxygen from the curing is the use of an antiwrinkle plastic sheet directly in contact with the composition to be cured. See U.S. Pat. No. 3,687,785. This sheet can be removed subsequent to the radiation step.
It is therefore a primary object of the instant invention to provide an apparatus and method to form a printing plate desirably having a tack-free surface. The basic underwater post exposure invention is described and claimed in copending U.S. Pat. application Ser. No. 843,391 entitled Photocuring Process for Eliminating Air Inhibition, filed Oct. 19, 1977, now abandoned and assigned to the same assignee as the present invention. Other advantages of secondary curing by the use of this invention also exists in providing a thoroughly cleaned and hardened printing plate face even when the surface might be substantially tack-free without the post exposure.